CHAPTER 4
LESSON 2
Earth’s Dynamic Surface
Shaping Earth’s Surface
Key Concepts
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before column
if you agree with the statement or a D if you disagree. After you’ve read
this lesson, reread the statements to see if you have changed your mind.
Before
Statement
After
3. Most earthquakes occur near tectonic plate
boundaries.
• Where do most earthquakes
occur?
• How are landforms related
to plate tectonics?
• Where do most volcanoes
form?
• How does plate movement
form mountains?
4. Volcanoes can erupt anywhere.
3TUDY #OACH
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Earthquakes
An earthquake is the vibrations caused by the rupture and sudden
movement of rocks along a break or a crack in Earth’s crust. On Earth,
Earthquakes occur every day. The strong shaking can damage
both natural features and human-made structures.
Create a Quiz Write a quiz
question for each paragraph.
Answer the question with
information from the
paragraph. Then work with a
partner to quiz each other.
Fault
Earthquakes can occur at faults. A fault is a crack or a fracture in
Earth’s crust along which movement occurs. One place where a fault
can exist is at a plate boundary.
Tectonic plates do not continually slide past each other
along faults. But, because of the convection currents beneath
the tectonic plates, forces build up along faults. Eventually,
these forces become so great that the rocks on either side
of the fault move and slide along the fault. When this happens,
the fault is said to rupture, and Earth’s crust moves along the
fault, causing an earthquake.
Where Earthquakes Occur
Most earthquakes occur at plate boundaries. Plate
boundaries are long and do not rupture all at once. Instead,
usually only small segments rupture. The amount of energy
an earthquake releases determines the size of the earthquake.
Scientists express the size of an earthquake in units of
magnitude.
Reading Essentials
Make a vertical three-tab
Venn book to compare and
contrast earthquakes and
faults.
Earthquake
Both
Fault
Earth’s Dynamic Surface
61
Earthquake Magnitudes
Interpreting Tables
1. Analyze How are fault
length and magnitude
related?
Magnitude
Average
Number per
Year
Typical Fault
Length on
Surface
Typical
Movement on
Fault
3
>100,000
15 m
1 mm
4
15,000
100 m
5 mm
5
3,000
800 m
3 cm
6
100
6 km
20 cm
7
20
40 km
1m
8
2
300 km
6m
Earthquake magnitude can range from less than one to at
least 9.9. As shown in the table above, small earthquakes
occur more frequently than large ones. Still, more than a few
major earthquakes occur each year.
2. Locate Where do most
earthquakes occur?
How Earthquakes Change Earth’s Surface
The movement of crust along faults can make mountains,
valleys, and other landforms. Different types of movement
occur at the three types of
Transform Boundary
plate boundaries.
Transform
fault
Transform Boundary Blocks of
Visual Check
3. Identify Circle the shift
in the stream caused by the
plate movement.
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Earth’s Dynamic Surface
crust move horizontally past
each other at a transform fault,
as shown in the figure to the
Continental
right. Both plate movement
crust
and earthquakes shift features
that cross the fault, such as
streams and roads. Transform
faults also are called strike-slip
faults.
Continental
crust
Lithosphere
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Key Concept Check
A plate boundary is made up of more than one fault. The
boundary covers a large region, and many smaller faults can
branch out from the main fault. Faults can be many
kilometers from the plate boundary. Earthquakes can occur
on these remote faults, just as they do on faults at plate
boundaries. Faults are largest where one plate subducts into
the mantle. The strongest and most damaging earthquakes
occur at these locations. Higher magnitude earthquakes
occur when movement along faults covers large distances.
In 2004, the convergent boundary between the Indian Plate
and the Burma Plate (part of the Eurasian Plate) ruptured.
That earthquake had a magnitude greater than 9.
Divergent Boundary Mid-ocean
ridges form between oceanic
plates, as illustrated to the right.
Between continental plates, one
side of the fault moves down
relative to the other side of the
fault. Normal faults form valleys
at divergent boundaries.
Divergent Boundary
Mid-ocean
ridge
Rift valley
Visual Check
4. Name What landforms
occur at divergent boundaries
between oceanic plates?
Oceanic
crust
Lithosphere
Asthenosphere
Convergent Boundary—
Subduction Zone The figure at left below shows what
happens to plates at a convergent boundary where
subduction occurs. The plate that does not subduct deforms
and crumples as the two plates push toward each other. As
the mantle near the subducted plate melts, magma rises and
forms a volcanic arc on the plate that does not subduct.
Convergent Boundary—No Subduction What happens at a
convergent boundary when subduction does not occur? The
edges of both tectonic plates crumple and deform, as shown
in the figure below on the right. Because neither plate
subducts, blocks of crust slide upward along a complex series
of faults called reverse faults. This results in the formation of
tall mountains.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Convergence
and Subduction
Deep ocean Volcanoes
trench
Convergence,
No Subduction
Key Concept Check
5. Describe How are
landforms related to plate
tectonics?
Mountains
Oceanic
crust
Lithosphere
Continental
crust
Continental
crust
Lithosphere
Visual Check
Asthenosphere
Asthenosphere
6. Point Out Circle the
landforms that develop at a
convergent boundary.
Volcanoes
The temperature inside Earth is high enough for rock to
melt. Geologists call molten rock stored beneath Earth’s surface
magma. Lava is magma that erupts onto Earth’s surface. Volcanoes
are vents in Earth’s crust through which molten rock flows. Volcanoes
are common on Earth. During the last 10,000 years, more
than 1,500 different volcanoes have erupted. Although they
are common, volcanoes do not form everywhere.
Reading Essentials
Earth’s Dynamic Surface
63
Where Volcanoes Occur
Reading Check
7. Explain How does
water released from
subducting rocks affect the
mantle?
Most volcanoes form at convergent plate boundaries.
Recall that at some convergent boundaries, one plate subducts
under another plate. Some rocks contain water within their
structure. As the rocks subduct, heat and pressure drive the
water out. This water can lower the melting temperature of
the mantle.
Magma then rises toward the surface and forms volcanoes
on the plate that does not subduct. A line of volcanoes forms
parallel to the plate boundary directly above the plate that
subducted. The volcanoes in Washington and Oregon, such
as Mount Rainier, Mount St. Helens, and Mount Hood,
formed above the subducting Juan de Fuca Plate.
How Volcanoes Change Earth’s Surface
Key Concept Check
8. Locate Where do most
volcanoes form?
Volcanoes are some of Earth’s most distinctive landforms.
Compared to other mountains, volcanoes can form quickly.
Mountains can form over millions of years, but volcanoes can
form in hundreds to thousands of years. Sometimes volcano
formation happens even more quickly. In Mexico, Paricutín
volcano grew 365 m above its surroundings within one year.
Reading Check
9. Describe How can
volcanoes change Earth’s
surface?
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Earth’s Dynamic Surface
At other times, volcanoes can erupt explosively. Much of
Mount St. Helens was destroyed during an eruption in 1980.
This kind of eruption can produce tiny pieces of glass made
from solidified lava. These pieces are called ash and can be
blown high into the atmosphere. When the ash falls back to
Earth’s surface, it can cover vast areas. Ash from Mount St.
Helens in Washington fell as far away as Minnesota and
Oklahoma.
Ocean Basins
You have read that lava erupts from volcanoes on land.
However, not all lava flows on land. Recall that the land
masses that make up North America and Europe began to
separate 200 million years ago when a divergent plate
boundary formed between them. What happened in the area
between these land masses as it became larger?
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Volcanoes erupt in two ways. Sometimes, lava can flow
over Earth’s surface before cooling, hardening, and becoming
solid rock. This is called a lava flow. Lava flows can be more
than 10 km long and over time can cover large areas
surrounding a volcano.
Lava on the Ocean Floor
Lava erupts at both convergent plate boundaries and
divergent plate boundaries. This lava hardens and forms new
crust. At an oceanic divergent plate boundary, magma rises
between two plates and forms new crust, as illustrated below.
As the plates move apart, more lava rises. The lava fills in
the space and forms more ocean crust. The seafloor between
North America and Europe is made of ocean crust that
formed after the continents began to spread apart.
Visual Check
10. Locate Highlight
where new ocean crust is
forming.
Formation of Oceanic Crust
Oldest
Pillow lava
Older
Older
Oldest
Youngest
Mid-ocean
ridge
Oceanic crust
Continental
crust
Continental
crust
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Magma
Asthenosphere
Asthenosphere
Mountains on the Ocean Floor
The ocean crust made at divergent plate boundaries is
not flat. Mid-ocean ridges are long, narrow mountains formed by
magma at divergent boundaries. The mid-ocean ridge in the
Atlantic Ocean extends through the middle of the Atlantic
Ocean, from near the North Pole to near the South Pole.
Mid-ocean ridges usually have gentle slopes and are about
2 km high.
Even though explosive volcanic eruptions usually occur
near convergent plate boundaries, more lava erupts at
divergent plate boundaries. Three-quarters of all lava erupts
at mid-ocean ridges. As lava erupts under water, it hardens
into flows and unique shapes, such as pillow lava.
Reading Essentials
Reading Check
11. Define What is a
mid-ocean ridge?
Earth’s Dynamic Surface
65
Mountains at Convergent Boundaries
Reading Check
12. Explain Why do most
mountains form near plate
boundaries?
Mountains form when Earth’s crust folds and crumples.
Where do you think this happens? Recall that tectonic plates
are rigid pieces of lithosphere. Collisions usually do not fold
the centers of these rigid plates. Instead, folding and
crumpling usually occur at the edges of plates. This is why
most mountains form near plate boundaries.
Recall that volcanoes form at convergent plate boundaries
where one plate subducts under another plate. These
volcanoes form volcanic mountain chains along the plate
boundaries. The Andes in South America and the Cascade
Range in North America formed this way.
When two continents collide at a convergent plate
boundary, large mountain ranges form. The tectonic plates
are under extreme pressure and fold or crumple upward. The
Himalayas formed as the Indian Plate converged with the
Eurasian Plate. The Himalayas are the largest and highest
mountain range in the world, and they are still growing.
Key Concept Check
The Appalachian Mountains in the eastern United States
and the Caledonian mountains in Scotland and Scandinavia
formed at the same convergent boundary. However, over
millions of years, tectonic plate motion broke up Pangaea. The
plates moved apart and the mountain chains separated.
13. Describe How does
66
Earth’s Dynamic Surface
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
plate movement form
mountains?
Reading Essentials
Mini Glossary
earthquake: the vibrations caused by the rupture and sudden
movement of rocks along a break or a crack in Earth’s crust
magma: molten rock stored beneath Earth’s surface
mid-ocean ridge: long, narrow mountains formed by magma
fault: a crack or a fracture in Earth’s crust along which
movement occurs
at a divergent boundary
volcano: vent in Earth’s crust through which molten rock flows
lava: magma that erupts onto Earth’s surface
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that
explains how an earthquake and a fault are related.
2. For each effect on landforms described in the diagram, identify the type of plate
boundary where the movement occurs. Choose from the list below.
convergent boundary, with subduction
divergent boundary
convergent boundary, without subduction
transform boundary
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Type of plate boundary
Effects on landforms
Volcanoes form.
Features that cross the fault, such as
streams, can shift.
Tall mountains form.
Valleys and mid-ocean ridges form.
What do you think
Reread the statements at the beginning of the
lesson. Fill in the After column with an A if you
agree with the statement or a D if you disagree.
Did you change your mind?
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END OF
LESSON
Earth’s Dynamic Surface
67